Mixtures, azeotropic partially miscible

We now consider systems which include a vapour phase together with liquid mixtures with partial miscibility. The first example is a system which shows partial miscibility and a minimum boiling point azeotrope. 

Azeotropic and Partially Miscible Systems. Azeotropic mixtures are those whose vapor and liquid equilibrium compositions are identical. Their x-y lines cross or touch the diagonal. Partially miscible substances form a vapor phase of constant composition over the entire range of two-phase liquid compositions usually the horizontal portion of the x-y plot intersects the diagonal, but those of a few mixtures do not, notably those of mixtures of methylethylketone and phenol with water. Separation of azeotropic mixtures sometimes can be effected in several towers at different pressures, as illustrated by Example 13.6 for ethanol-water mixtures. Partially miscible constant boiling mixtures usually can be separated with two towers and a condensate phase separator, as done in Example 13.7 for n-butanol and water. 

Figure 13.28. Vapor-liquid equilibria of some azeotropic and partially miscible liquids, (a) Effect of pressure on vapor-liquid equilibria of a typical homogeneous azeotropic mixture, acetone and water, (b) Uncommon behavior of the partially miscible system of methylethylketone and water whose two-phase boundary does not extend byond the y = x line, (c) x-y diagram of a partially miscible system represented by the Margules equation with the given parameters and vapor pressures Pj = 3, = 1 atm the broken line is not physically significant but is...

At atmospheric pressure, the n-butanol-water system exhibits a minimum boiling azeotrope and partial miscibility, and hence a binary heterogeneous azeotrope. Figure 1.8 shows the Tyx and Pyx phase diagrams for l-propanol(l)-water(2) azeotropic mixture obtained from the Aspen Plus simulator using the NRTL activity coefficient model. 

Separation of a Partially Miscible Mixture Water and n-butanol in the concentration range of about 50-98.1 mol % water form two liquid phases that boil at92.7°C at one atm. On cooling to 40°C, the hetero-azeotrope separates into phases containing 53 and 98 mol % water. 

The constant vapor pressure (boiling temperature) above the two-phase region of certain partially miscible mixtures is usually larger (smaller) than the vapor pressure (boiling temperature) at any other liquid-phase composition in the homogeneous region. In this case, the vapor-phase composition is inside the miscibility gap. Mixtures of this type are called heteroazeotropic mixtures, or simply heteroazeotropes. (Fig. 1, Type II), as opposed to the other types of azeotropes, called homoazeotropes. 

Only in a few cases partially miscible mixtures present a positive or negative azeotropic point in the single-phase region, outside the miscibility gap, similar to the azeotropic points of homogeneous mixtures (Fig. 1, Types IV and VI). 

The use of limiting activity coefficients for the prediction of azeotropic compositions at constant temperatures and of the partial miscibilities in binary liquid mixtures has been reported by Brandani [58 a]. 

Some partially miscible mixtures cannot be separated directly by this method. For example, methyl ethyl ketone and water are pai tially miscible and form an azeotrope at atmospheric pressure, but... 

The enthalpy of mixture for this system is not available in the literature. Only the phase diagram is given [14, 15]. The phase diagram presents a eutectic point at x(NdF3) = 0.69 and T = 1488 K (Figure 3.5.1). This figure shows a partial miscibility in a solid state and an azeotrope at x(NdF3) = 0.21 and T = 1688 K. 

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